The composition of the gas stream emanating from an automobile airbag inflator is subject to strict requirements to avoid toxicity concerns. Generally, solid propellant gas generators produce unacceptable byproducts which must be removed from the gas stream prior to exiting the gas generator. Due to the high temperatures involved in burning solid propellants, many of the unacceptable byproducts are in the form of liquids or gases which are difficult to remove unless the gases are cooled to the point where the undesirable byproducts convert to solids which can be filtered out or to liquids which solidify in contact with cool surfaces of the gas generator.
The conventional approach to solving the aforesaid problem has been to direct the hot propellant gases directly into a coolant/filter mass and rapidly cool the gases down in a single step to the point where the undesirable solid liquid byproducts are removed. However, a problem is presented by this approach in that rapid cooling of the gases may stabilize the propellant combustion products in a manner that leads to unacceptably high levels of undesired gases.
For example, in airbag inflators, low levels of NO and CO in the-effluent gases are mandated. When a stoichiometric propellant containing N, C and Q is burned, the quantity of NO and CO produced is a function of the propellant combustion temperature. More Co and NO are formed at higher temperatures. If as in a conventional system, the combustion gases are quenched in a single step to a temperature at which the gas reaction rates are reduced to essentially zero, unacceptably high CO and NO levels resulting from the resultant propellant combustion temperature equilibrium condition may be produced.
The inventive concept of the present invention is to solve the aforesaid problem by using a gas generator which incorporates staged cooling and filtering to achieve the desired gas outlet properties. With sufficient residence time between stages, the CO and No equilibrium condition can be shifted to relatively low, acceptable, CO and NO concentrations. In one embodiment of the invention, cooling of the propellant combustion gases in stages is achieved by alternate radial and circumferential flow of gas. However, the concept is also applicable to axial flow gas generators.
More, specifically, upon receipt of an initiation signal, an initiator or squib ignites a solid propellant which burns rapidly, evolving gas, liquid and solid byproducts. The evolving propellant gases pressurize the inside of a propellant tube. When internal pressure in the propellant tube reaches a predetermined level, a burst foil on the inside thereof ruptures allowing the propellant gases to flow radially outwardly through the perforated wall of the propellant tube, thence through slagging screens into a surrounding plenum defined by a baffle tube. Initial cooling of the combustion products to an intermediate temperature is achieved as the combustion products pass through the slagging screens. High freezing temperature and solid byproducts are initially filtered out by the slagging screens. The gases are further cooled in the plenum by heat transfer to the baffle tube wall. Additional solids removed is achieved-by impact plating on the radially inner wall of the baffle tube. The size of the plenum is designed to provide a sufficient average residence time such that the CO and NO concentrations can shift toward relatively low equilibrium concentrations associated with lower gas temperature within the plenum.
The propellant gases then exit the plenum by first moving circumferentially then radially through axially aligned relatively large openings in the surrounding baffle tube. The propellant gases then enter cooling screens wrapped around the baffle tube and flow circumferentially in opposite directions through the screens to exhaust orifices located on the opposite side of the generator housing from the openings in the baffle tube.
As the gases flow through the aforesaid paths they are cooled relatively slowly to induce shifting of CO and NO to even lower concentrations. The coolant screens cool the gases to a desired exit temperature and filter out any remaining condensible liquid or solid propellant byproducts that may be entrained therein.
The propellant gases then exhaust radially from the gas generator via orifices in the generator housing. Depending upon ballistic requirements, the orifices may or may not be sealed with a burst shim.